1. Field of the Invention
This invention relates to an improvement of the process for producing sintered iron ore with a Dwight-Lloyd machine, and more particularly to a process for producing sintered iron ore having high reducibility as achieved by noting the moisture content of the raw material mix prior to ignition.
2. Description of the Prior Art
To produce sintered iron ore with a Dwight-Lloyd machine, several sintering materials are first combined with a mixer, water is added to the raw material mix, the mix is granulated, the granules are laid down on the pallets of the machine, and are fed to the ignition furnace where the surface of the granule bed is uniformly ignited. The ignited layer lowers down as it is burned with air sucked through the mix bed toward the underside of the pallets; therefore, the sintering reaction proceeds gradually from the surface layer of the mix bed to the lower layer. For the burning layer to proceed quickly and to obtain sintered iron ore having high porosity and reducibility (JIS M 8713) (Japanese Industrial Standard M 8713), it is necessary that ignition bring part or all of the mix bed to a sintered state, therefore the sinter bed must have good permeability to air. Accordingly, efforts are being made to increase the productivity of sintering process by enhancing such permeability, to minimize the use of coke and coke oven gas (COG) necessary for production of sintered iron ore, and to better the quality of the sintered product. The quality of sintered iron ore is commonly expressed by such indices as tumbler index (TI) and shatter index (SI) for indicating the strength at ordinary temperature and degradation index after 550.degree. C. reduction (RDI) for representing the ease of degradation in a reducing atmosphere at relatively low temperature (550.degree. C.). Although attempts have been made to improve these indices, the recent studies on the reduction of the ferrous feed in the blast furnace show the importance of a property at high temperatures such as the reducibility (determined at 900.degree. C. according to JIS M 8713), in addition to TI, SI and RDI as the indices for the quality of sintered iron ore.
Several examples of the prior art processes for providing a sintered ore product with improved quality are described below.
(1) In Blast Furnace Coke Oven and Raw Materials Conference (1960) pp. 409-428 "Use of a Preheated Air in Sintering", D. F. Ball and J. M. Ridgion report the technique of drying preliminarily the raw material mix with hot air prior to ignition. The data given in this article shows that about 70% of moisture is removed from the mix by the preliminary drying which continues for a period equal to the sintering time.
(2) Stahl und Eisen, 97 (1977) 7, 309 discloses the technique of preheating the mix with a low-oxygen gas to a temperature higher than the ignition point of coke to thereby complete dehydration, calcination and reduction steps prior to sintering. The technique aims at complete removal of the moisture content of the mix prior to ignition. Since the article states that the preheating is advantageously effected at 800.degree. C. for a period of about 10 to 15 minutes, this prior art technique requires extended preheating and uses a great amount of energy as a heat source for preheating.
Therefore, the above two prior art techniques that perform preliminary heating of the mix before ignition require a long time to achieve the intended purpose, and in particular, the technique reported in Stahl und Eisen supra that finishes dehydration, calcination and reduction before sintering requires a large volume of heated non-oxidative gas, and therefore, either technique is very difficult to implement with a Dwight-Lloyd machine.
(3) Japanese Patent Public Disclosure No. 52903/76 discloses the technique of elevating only the upper layer of the mix (the top layer 30 mm deep from the surface) to about 200.degree. C. and igniting the layer with reduced thermal impact upon ignition. But this technique does not pay attention to the moisture content of the mix before it is laid down on pallets or to the moisture content of every part of the mix from the top layer to bottom layer after it is laid down on the pallets. The technique intends to decrease the resistance to air permeation of the mix at the top layer to thereby enhance the productivity of the sintering process as well as to reduce the coke consumption, but it does not at all consider the possibility of improving the characteristics of the sintered ore product at high temperatures.